1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, to an in-plane switching mode LCD device, and a method for fabricating the same.
2. Discussion of the Related Art
Recently, with the increase of the need for displaying information and the demand for using portable information systems, light and thin film type flat panel display (FPD) devices have been actively researched and commercialized, and the conventional cathode ray tube (CRT) devices have been replaced. Among these flat panel display devices, an LCD device is utilized for displaying an image by utilizing an optical anisotropy of a liquid crystal. The LCD device can be installed in a notebook computer, a desktop monitor, or any other display devices because of its excellent resolution, color rendering capability and picture quality.
A method for driving the LCD device includes a twisted nematic (TN) method that drives a liquid crystal molecule on a nematic in a vertical direction to a substrate. However, this method has a problem that a viewing angle is as narrow as 90°. This is due to a refractive anisotropy of the liquid crystal molecule by which the liquid crystal molecule disposed parallel to the substrate is aligned in the vertical direction to the substrate when a voltage is applied to a liquid crystal display panel.
Accordingly, an in-plane switching method has been proposed to improve the viewing angle to more than 170° by driving the liquid crystal molecule in a horizontal direction for the substrate. The in-plane switching method will be explained in more detail.
FIG. 1 is a plane view showing a part of an array substrate for a conventional in-plane switching LCD device, in which only one pixel is shown by way of illustration. In fact, there are N gate lines and M data lines crossing each other, and therefore N×M pixels exist in an actual LCD device.
As shown in FIG. 1, an array substrate 10 includes a gate line 16 and a data line 17 for defining a pixel region by being arranged horizontally and vertically, a thin film transistor 20 formed at the intersection region between the gate line 16 and the data line 17 as a switching device, and a pixel electrode 18 formed at each pixel region.
The thin film transistor 20 includes a gate electrode 21 connected to the gate line 16, a source electrode 22 connected to the data line 17, and a drain electrode 23 connected to the pixel electrode 18. The thin film transistor 20 further includes first and second insulating layers (not shown) for insulating the gate electrode 21, and the source and drain electrodes 22, 23, and an active layer, namely, a channel layer (not shown) for forming a conductive channel between the source electrode 22 and the drain electrode 23 by a gate voltage supplied to the gate electrode 21.
In the pixel region, a common electrode 8 and the pixel electrode 18 for generating an in-plane horizontal electric field are alternately disposed. The common electrode 8 is diverged from a common electrode line 8a disposed parallel to the gate line 16. The pixel electrode 18 is electrically connected to the drain electrode 23, and diverged from a pixel electrode line 18a overlapped with the common electrode line 8a. The common electrode line 8a is formed on the same plane as the gate line 16. The pixel electrode line 18a is formed on the same plane as the data line 17. An insulating layer is interposed between the common electrode line 8a and the pixel electrode line 18a thus to form a storage capacitor.
In the LCD device, an amorphous silicon thin film was mainly utilized as the channel layer of the thin film transistor 20. The amorphous silicon thin film transistor technique was first described by English LeComber et al. in 1979, and commercialized as a 3-inch liquid crystal portable television in 1986. Recently, an amorphous silicon thin film transistor LCD device with a large area of more than 50-inch has been developed.
However, the field effect mobility of the amorphous silicon thin film transistor of about (<1 cm2/Vsec) prevents its use in peripheral circuits that apply signals to the pixel region, because the peripheral circuits operate at more than 1 MHz. Accordingly, the research and development have been actively performed to simultaneously form a switching transistor in the pixel region and peripheral circuits in a driving circuit region together on a glass substrate by utilizing a polycrystalline silicon thin film transistor, which has the field effect mobility greater than that of the amorphous silicon thin film transistor.
The polycrystalline silicon thin film transistor technique has been applied to a small module such as a camcorder etc. since a liquid crystal color television was developed in 1982. Since the polycrystalline silicon thin film transistor has low photosensitivity, high electric field effect and mobility, a driving circuit can be directly fabricated on a substrate.
Increased mobility enhances the operation frequency of the driving circuit that determines the number of driving pixels that can be driven while maintaining an adequate display capability. More specifically, the increased frequency decreases the charging time of a signal applied to a pixel such that distortion of the signal is decreased and picture quality is thereby improved.
Accordingly, the conventional additional process for connecting a driver integrated circuit (IC) and the pixel array is not necessary thus to increase productivity and reliability. In addition, since the polycrystalline silicon thin film has the above-mentioned excellent characteristics, a smaller and excellent thin film transistor can be fabricated.
The polycrystalline silicon thin film generally has the coplanar structure, in which the gate, source and drain electrodes are formed on the same layer on the basis of the active layer. The active layer is mainly positioned at the lowest layer of the substrate.
In the thin film transistor of the coplanar structure, the channel layer is exposed to lower backlight. Therefore, the channel layer is electrically and optically influenced by the backlight, which may cause an off-current, namely, a leakage current to be increased thus to deteriorate characteristics of the device and lower image quality of an LCD panel.
In order to solve the above-mentioned problem, a method has been proposed to additionally dispose a light-shielding layer below the channel layer to shield incident light thereonto. However, the fabrication process becomes complicated because an additional process is required for forming the light-shielding layer.
In addition, the number of photolithography processes are required in the thin film transistor of the coplanar structure utilizing the polycrystalline silicon thin film more than in a staggered structure, thereby increasing fabrication costs.